Squeaks happen: A little grease might do the trick

Leah Karibian
Regional Engineering Manager
Nye Automotive
Troy, Mich.

Edited by: Amy Higgins

Drivers expect smooth, quiet performance from their cars. The least little squeak or barely audible rattle can send some of them around the bend. Automakers recognize this, and to keep customers satisfied have created engineering groups dedicated solely to tackling buzzes, squeaks, and rattles (BSR).

BSR engineers know that eliminating noise and vibration doesn't necessarily mean expensive redesign or tight engineering tolerances. Sometimes, putting the right lubricant in the right place does the job more effectively and more economically.

Knock, knock
Visteon Automotive Systems, for instance, was concerned with a knocking noise in its rack-and-pinion steering system noticeable under mechanical shock, as over potholes or railroad tracks. Functionally, the steering system consists of a toothed rack that mates to a pinion gear. When the steering wheel turns, the gear rotates, moving the rack left or right to steer the car.

While Visteon wanted a grease that prevents wear and extends the operating life of the gear, it was also concerned about this separate wear and noise problem. On the smooth side of the rack, a spring-loaded yoke keeps rack teeth mated to the pinion gear. The yoke's inserted through a tapped hole on the rear of the rack-and-pinion housing. A plug, screwed into the hole, compresses the spring and holds the yoke in place. Under mechanical shock the rack bounces and jars the yoke, which causes a knocking sound and intensifies wear.

Engineers traditionally solve this dilemma by inserting an expensive, composite plastic sleeve between the yoke and the polished side of the rack to reduce wear on the rack and yoke. Plastic sleeves, however, wear quickly if the proper amount of torque isn't applied to the yoke plug. Too much torque on the yoke plug speeds wear due to excessive pressure; yet not enough torque accelerates wear because there is too much play between the yoke and the rack. The margin for error is thin and the tolerance can be too tight to ever know the exact measure.

Visteon found a solution with a custom-formulated, synthetic grease. The grease consists of a new, high-viscosity base oil, lubricious thickening ingredients, and extreme pressure and antiwear additives. The viscous grease adheres to moving parts, so mating surfaces don't touch, they actually move within the grease. This eliminates noise at the yoke/rack interface, and the need for inserts between the yoke and rack. In addition, the grease permits greater tolerances when securing the yoke plug.

When grease is applied to gear teeth and between the yoke and the smooth side of the rack, the rack-and-pinion unit passes gear and yoke wear tests without knocking. Visteon reports a smooth, quality feel across the whole steering system. Visteon's quiet rack-and-pinion system will be in next year's Mazda. Ford will also lubricate rack-and-pinion systems with the new grease for the Lincoln LS, Thunderbird, and Jaguar as soon as dispensing equipment on production lines is converted. In addition to longer life and quieter performance, the Mazda rack-and-pinion system uses only 12 gm of synthetic grease, compared to 50 gm of petroleum grease formerly used on the device.

The new grease gave Visteon more than it bargained for. Visteon uses two yoke styles, an oval and Y-shape. After heat treating the Y-racks, Visteon traditionally hand-buffed each rack to remove scaling and aspirates to reduce wear. A Nye engineer suggested that Visteon wear-test unpolished racks lubricated with the synthetic grease. Results showed that unbuffed racks outperformed buffed racks. This lets Visteon abandon labor-intensive, rack polishing of the more than 600,000 Y-racks it manufactures each year.

From leaf springs to radio tuners
Leaf springs are one of the oldest and most reliable suspension systems used today. Widely used in the heavy-car 60s and 70s, they are now found mostly on larger vehicles, pick-up trucks, and SUVs. Rugged, economical, and fairly inexpensive to repair, this steel-on-steel component is, however, predisposed to pesky BSR problems. In a marketplace that wants even its pick-ups quiet, DaimlerChrysler tackles the problem head-on.

Generally, leaf-spring squeaks are caused by friction. So, manufacturers insert plastic strips between the steel layers of some leaf springs to damp metal-on-metal noise, and reduce metal wear. But even with fortified-nylon inserts laced with slippery PTFE, squeaks happen.

Lubricants usually cut friction, but DaimlerChrysler also tested whether or not they further damp noise. A leaf-spring lubricant must be a heavy, sticky grease to stay in place on plastic inserts, especially under heavy load and shock conditions. It also must resist dirt, water, and saltwater washout. These operating conditions suggest an extremely viscous "damping grease."

Damping greases were first formulated some 50 years ago to economically build fine tolerances into microscopes, telescopes, and binoculars. They work because they are formulated with highly viscous base oils. These oils give damping greases high internal shear resistance. This prevents backlash and coasting and ensures smooth, incremental motion. To get the "right feel" for a specific component, engineers can choose from various grease consistencies. For example, the volume control on a radio calls for a lighter grease while the release mechanism on a parking brake requires heavier grease. Due to their consistency, damping greases also seal out moisture, dust, and other pollutants.

In the mid-1980s, the first line of broad-temperature damping greases caught the attention of automotive switch manufacturers. When damping grease is applied to detents, it curbs the sometimes annoying click plastic switches can make. It also gives a tactile, "velvet feel" and makes precise settings possible that could not otherwise be made by hand. Today, damping greases are used in more than 30 automotive parts including DaimlerChrysler's leaf-spring suspension systems.

DaimlerChrysler tested an ultraviscous damping grease fortified with molybdenum disulfide to boost load-carrying capability on its leaf springs. The grease cut noise and significantly reduced leaf-spring-related warranty issues from the field. Ford specifies an ultralight version of this grease in HVAC and radio controls. General Motors uses a medium grade to quiet rattles in steering columns.

The right damping grease can likely solve other common noise and motion problems, including recoil speed of foot-pedal parking brakes, temperature-control cables that don't slide smoothly, rearview mirrors that drift when the car is in motion, and shifter cables that carry vibration from the transmission to an operator's hand resting on the shift stick. While all of these components may be defect-free and meet manufacturer's life requirements, if they don't have the "right feel" they can be perceived by vehicle owners as something less than a quality component. Damping greases are a way to design into the component the tactile and acoustic qualities consumers want.

Brakes and starters
Silicone greases have a good track record for extending the life and eliminating noise in push-pull cable applications. While oil is still used for some lightly loaded cables with high-efficiency needs, a grease's stay-in-place capability lessens leakage associated with oil. Greases also prevent wear better than oils. Formulated by gelling a base oil with a thickener system, greases act like a sponge, slowly releasing oil throughout the life of the cable and supplying adequate lubricant during cycling.

PTFE is a popular gellant for cable greases because it creates a surface with a low coefficient of friction and supports medium and heavily loaded cables to curtail wear. Because grease is a less mobile form of lubrication it can also damp mechanical vibration from one end of the cable to the other and deliver a "quality feel." Both Ford and General Motors specify a soft, high-viscosity silicone fluid gelled with PTFE for parking brake cables in light trucks. DaimlerChrysler specifies a medium silicone-based grease for parking brake cables, and adds a tube of the grease to its Mopar BSR kit.

Gearbox manufacturers often use oil to quiet noise and extend gear life because of its wetability. Another option, however, is to use a soft grease because it helps quiet the gearbox and cuts design and manufacturing costs associated with oil seals. Delco Remy starters, for instance, rely on such a grease; a special blend of lithium soap and synthetic hydrocarbon and ester oils with an operating temperature range of 40 to 135°C. The grease actually softens to the consistency of soft butter as it is churned by gears. And, as the grease softens, its wetting ability increases and retards friction and wear, while its gellike consistency helps reduce noise. The grease has an EP additive package, including molybdenum disulfide, making it suitable for metal-on-metal and high-load gearboxes. Moreover, the grease won't "channel."

Oftentimes, OEMs that switch from oil to grease in the gearbox opt for the same grease used to lubricate the gearmotor bearings. But, bearing greases are designed to channel, that is to move out of the way of the rolling element so it rotates freely on a thin layer of oil. While that works for bearings, it can be disastrous for gears often resulting in metal-on-metal contact, noise, and premature failure.

Squealing motors
Sometimes, it takes more than a lubricant to get rid of noise. A Tier-One supplier wanted an oil for powdered metal bearings in an HVAC motor. More than preventing wear, they wanted an oil to silence a car heater that started squealing at about 10°F. The customer sent "dry," or unlubricated bearings to Nye for preapplication testing. Through a soxlet extraction process, Nye engineers removed oil from the dry bearings — not lubricating oil, but a residual process oil used in manufacturing sintered bearings. Process oil left in the bearings can take up space, reducing the amount of lubricant the bearing can hold. Also, if the lubricant of choice is a polyalphaolefin, ester, or polyglycol, process oils with their poor thermooxidative stability can further degrade the lubricant.

Nye, therefore, recommends more than a lubricating oil for sintered bearings. In fact, it suggests a complete impregnation process: extract process oils, use a perfluoropolyether (PFPE) oil to assure good low-temperature performance, and ensure a full complement of oil within the bearing by impregnating in a vacuum chamber for 24 hr at 100°C. The result: the low-temperature squeal is squelched and the motor's operating life is extended.

PFPE lubricants offer the widest operating temperatures of any synthetic lubricant: 90 to 250°C. They are used primarily on automotive assembly lines because they won't crack, craze, discolor, or dissolve plastic or cause natural rubber or elastomers to swell, shrink, or become brittle. Additionally, PFPEs have no adverse effect on metal, and when used on weather stripping, they eliminate squeaking dry rubber. The lubricants are also often used in passenger compartments for plastic control knobs and hinges, and many other last-minute squeaks.